Marius is one of those names I really want to like a lot, but instead I have a rather mediocre perspective about it. It sounds too much like 'marry us' to my ear. If I met someone with this name, I'd probably think it was kinda cool, but it's not a name I'd ever choose myself.
As far as middle names go, I think a classic, solidly masculine name would probably work best, perhaps of the one syllable variety.

I'm a huge Marius fan. It's strong and romantic with a wonderful continental flair. I don't mind the Les Mis association, but it's not why I love the name. I do love Les Mis, but I do think the name might be a bit more usable (in my mind) if the character were not also in Les Mis. In any case, Marius is definitely on my long list of boys names. You can even see him in my signature below! As you can see, I have Marius paired with Owen right now. My creative juices aren't flowing right now, but I'll continue to try to think of more.